A conventional method of preparing hydroxypropyl methylcellulose acetate succinate (HPMCAS) includes preparing a reaction solution by esterifying three types of reactants (i.e., hydroxypropyl methylcellulose, acetic anhydride, and succinic anhydride) in a reaction medium such as acetic acid in the presence of a catalyst such as a sodium acetate, and then forming HPMCAS grains by adding purified water to the reaction solution at a predetermined ratio.
However, HPMCAS powder prepared according to the method has a low fraction of grains having an appropriate grain size range and is dissolved in a solvent with a low dissolving velocity.